Memory devices are typically provided as internal, semiconductor, integrated circuits in apparatuses such as computers or other electronic devices. There are many different types of memory including non-volatile memory and volatile memory. Volatile memory can include memory such as static random-access memory (SRAM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM). Non-volatile memory can include memory such as read only memory (ROM), phase change memory (PCM), flash memory, or ferroelectric random access memory (FeRAM).
FeRAM is a random-access memory that is similar in construction to DRAM (e.g., each cell having a capacitor and access transistor) but uses a ferroelectric layer instead of a dielectric layer in order to achieve non-volatility. An FeRAM cell can comprise a dielectric structure that includes a non-linear ferroelectric material (e.g., lead zirconate titanate (PZT)). The ferroelectric material typically has a higher dielectric constant than that of a DRAM's linear dielectric material due to the effects of semi-permanent electric dipoles formed in the crystal structure of the ferroelectric material.
One ferroelectric characteristic is that the ferroelectric material has the form of a hysteresis loop that is similar in shape to the hysteresis loop of ferromagnetic materials. When an external electric field is applied across a dielectric, the dipoles tend to align themselves with the field direction, produced by small shifts in the positions of atoms and shifts in the distributions of electronic charge in the crystal structure. After the charge is removed, the dipoles retain their polarization state. Binary “0”s and “1”s can be stored as one of two possible electric polarizations in each data storage cell.
A write operation to an FeRAM cell is similar to a DRAM write operation. A field is applied across the ferroelectric layer by charging the plates on either side of it, forcing the atoms inside into the “up” or “down” orientation (depending on the polarity of the charge), thereby storing a binary “1” or “0”. A sense operation, however, is somewhat different than a DRAM in that the transistor forces the cell into a particular state (e.g., “0”). If the cell already held that particular state, nothing will happen in the output lines. If the cell held an inverse state (e.g., “1”), the re-orientation of the atoms in the film will cause a brief pulse of current in the output as they push electrons out of the metal on the “down” side. The presence of the residual charge on a coupled data line (e.g., bit line) can indicate that the cell held the inverse state. Since this process overwrites the cell, sensing FeRAM is a destructive process and the cell should be re-written if it was sensed.
FeRAM sensing schemes use a ramped plate voltage on the FeRAM plate in order to sense the residual charge left on the data line. As the plate voltage ramps upward, a polarized memory cell pushes its charge onto the data line, thus resulting in two different data line voltages depending on whether the memory cell was polarized. While memory manufacturers typically would like to reduce power consumption and increase memory performance, applying a ramped plate voltage can be a time and energy intensive process.
Thus, there are general needs to be able to sense FeRAM with a faster and lower energy process.